U.S. patent application number 11/719437 was filed with the patent office on 2009-05-21 for enamel composition, assembly and use thereof on a substrate surface.
This patent application is currently assigned to FERRO TECHNIEK HOLDING B.V.. Invention is credited to Simon Kaastra, Rudi Meinen.
Application Number | 20090130470 11/719437 |
Document ID | / |
Family ID | 34974523 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090130470 |
Kind Code |
A1 |
Kaastra; Simon ; et
al. |
May 21, 2009 |
ENAMEL COMPOSITION, ASSEMBLY AND USE THEREOF ON A SUBSTRATE
SURFACE
Abstract
The invention relates to an enamel composition for application
in an enamel layer in heating elements. The invention further
comprises an assembly of such an enamel composition and a substrate
surface. In addition, the invention comprises a heating element
comprising such an assembly. The invention moreover comprises the
use of such an enamel composition for applying an enamel layer to a
substrate surface. The enamel composition makes possible an
electrical heating element with improved durability and increased
safety.
Inventors: |
Kaastra; Simon; ( Dinxperlo,
NL) ; Meinen; Rudi; (Aalten, NL) |
Correspondence
Address: |
RENNER OTTO BOISSELLE & SKLAR, LLP
1621 EUCLID AVENUE, NINETEENTH FLOOR
CLEVELAND
OH
44115
US
|
Assignee: |
FERRO TECHNIEK HOLDING B.V.
AT Gaanderen
NL
|
Family ID: |
34974523 |
Appl. No.: |
11/719437 |
Filed: |
November 23, 2005 |
PCT Filed: |
November 23, 2005 |
PCT NO: |
PCT/NL2005/050050 |
371 Date: |
December 15, 2008 |
Current U.S.
Class: |
428/469 ;
106/286.5 |
Current CPC
Class: |
C03C 8/20 20130101; Y10T
428/263 20150115; C03C 8/10 20130101; C03C 8/16 20130101; C03C
2207/04 20130101 |
Class at
Publication: |
428/469 ;
106/286.5 |
International
Class: |
B32B 15/04 20060101
B32B015/04; C09D 1/00 20060101 C09D001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 23, 2004 |
NL |
1027571 |
Feb 11, 2005 |
NL |
1028258 |
Claims
1. Enamel composition for application in an enamel layer in heating
elements, comprising between 0 and 10% by mass of V.sub.20.sub.5,
between 0 and 10% by mass of PbO, between 5 and 13% by mass of
B.sub.20.sub.3, between 33 and 53% by mass of Si0.sub.2, between 5
and 15% by mass of A1.sub.20.sub.3, and between 20 and 30% by mass
of CaO.
2. Enamel composition as claimed in claim 1, characterized in that
the mass of V.sub.20.sub.5 and PbO is a maximum of 12% by mass.
3. Enamel composition as claimed in claim 1, characterized in that
the quantity of V.sub.2O.sub.5 lies between 0 and 2% by mass.
4. Enamel composition as claimed in claim 1, characterized in that
the enamel composition is substantially free of lithium and sodium
ions.
5. Enamel composition as claimed in claim 1, characterized in that
the enamel composition comprises between 0.1 and 6% by weight of
potassium.
6. Assembly of a substrate surface and an enamel layer of an enamel
composition as claimed in claim 1 applied to the substrate
surface.
7. Assembly as claimed in claim 6, characterized in that the
substrate surface is manufactured substantially from metal.
8. Assembly as claimed in claim 6, characterized in that the
substrate surface is manufactured from a material with a
coefficient of expansion which differs by between 20 and 45% from
the coefficient of expansion of the enamel layer.
9. Assembly as claimed in claim 6, characterized in that the
substrate surface substantially consists of ferritic chromium steel
with a chromium content of at least 10% by mass.
10. Assembly as claimed in claim 6, characterized in that the
enamel layer applied to the substrate surface is provided on the
side remote from the substrate layer with a second enamel layer,
wherein the second enamel layer has a relatively higher
concentration of alkali metal ions than the enamel composition of
the enamel layer applied to the substrate surface.
11. Assembly as claimed in claim 10, characterized in that the
enamel layer applied to the substrate surface is substantially free
of alkali metal ions.
12. Heating element comprising an assembly as claimed in claim
6.
13. Use of an enamel composition as claimed in claim 1 for applying
an enamel layer to a substrate surface of a heating element.
Description
[0001] The invention relates to an enamel composition for
application in an enamel layer in heating elements. The invention
further comprises an assembly of such an enamel composition and a
substrate surface. In addition, the invention comprises a heating
element comprising such an assembly. The invention moreover
comprises the use of such an enamel composition for applying an
enamel layer to a substrate surface.
[0002] The use of an enamel layer as dielectric in the manufacture
of, among other products, electrical heating elements is known, for
instance from WO 00167818. Heat is generated by conducting electric
current through an electrical resistance arranged on an enamel
layer, for instance in order to heat liquids. The dielectric herein
provides for electrical insulation of the electrical resistance
which generally consists of a metal track. The manufacture of the
dielectric from enamel herein results in a mechanically relatively
strong dielectric which conducts heat relatively well. The
composition of the enamel for application as dielectric is however
critical in order to enable optimization of the electrical
properties, particularly at temperatures occurring during use. The
specific electrical resistance of the dielectric is generally high
at room temperature, usually higher than 10.sup.12
.OMEGA..quadrature.cm, and fills sharply as temperatures increase
in the order of magnitude of 10.sup.5 .OMEGA..quadrature.cm at
400.degree. C. At such a resistance a (relatively small) leakage
current through the dielectric becomes possible. Detection of the
leakage current through the enamel layer can provide relevant
information in respect of the temperature of the heating element,
and can be utilized in a circuit to prevent overheating of the heat
element, as is known from WO 0167818.
[0003] Another property which determines the quality, and thereby
the applicability, of the dielectric is the breakdown voltage.
Breakdown voltage is the level of the electrical potential
difference over the dielectric at which electric current flows
through the dielectric. This can result in undesirable adverse
effects, and even the irreparable disintegration of the enamel
layer. In order to guarantee maximum safety in an electrical
heating element, the breakdown voltage must therefore be high,
preferably at least 1250 VAC (alternating current). There is a need
for heating elements in which such a safety level can be realized
in simple manner. The breakdown voltage of the dielectric is
determined by multiple factors, including among others the layer
thickness of the dielectric, the enamel composition and structural
defects such as gas bubbles present in the dielectric. A good
adhesion of enamel to the substrate (generally steel or aluminium)
is also important here.
[0004] There exists a need for enamel compositions with improved
durability when used in heating elements.
[0005] The invention has for its object to provide such an enamel
composition.
[0006] The invention provides for this purpose an enamel
composition for application in an enamel layer in heating elements,
comprising between 0 and 10% by mass of V.sub.2O.sub.5, between 0
and 10% by mass of PbO, between 5 and 13% by mass of
B.sub.2O.sub.3, between 33 and 53% by mass of SiO.sub.2, between 5
and 15% by mass of Al.sub.2O.sub.3, and between 20 and 30% by mass
of CaO. Such a composition produces an enamel layer with an
improved durability when used in heating elements. The enamel
composition can be melted relatively easily, and herein has a
favourable viscosity whereby it can be readily applied to different
types of surface. The enamel composition adheres particularly well
to metals, in particular steel and more particularly to ferritic
chromium steel 1.4521. The maximum compressive stress of the enamel
layer obtainable from the enamel composition lies for the new
composition in the range 2.0-2.5.times.10.sup.8 Pa. For known
enamel compositions the maximum compressive stress generally lies
in the range 0.7-1.7.times.10.sup.8 Pa. The enamel composition
according to the invention also has a better temperature
resistance, so that prolonged exposure to temperatures up to
600.degree. C., with peak loads of 750.degree. C., presents no
problem. The enamel composition is less susceptible to degeneration
due to prolonged load at a high voltage than known enamel
compositions. The rheological properties of the enamel composition
are furthermore such that the enamel composition can be applied
relatively simply to a substrate, wherein the chance of crack
formation in the case of temperature changes is also reduced. Such
an enamel composition is particularly suitable as dielectric in
electrical heating elements. It is noted for the sake of clarity
that the content of both PbO and V.sub.2O.sub.5 lies between 0 and
10% by mass, which thus implies that both PbO and V.sub.2O.sub.5
will be present in any embodiment variant of the enamel composition
according to the invention so as to be able to impart the above
stated advantageous properties to the enamel composition.
[0007] The mass of V.sub.2O.sub.5 and PbO is preferably a maximum
of 12% by mass. This results in an enamel composition which can be
processed relatively well as a paste, wherein an enamel layer is
obtainable with a good adhesion to metal surfaces.
[0008] It is recommended that the quantity of V.sub.2O.sub.5 lies
between 0 and 2% by mass. A sufficiently adhesive enamel
composition is thus obtained with favourable processing properties,
wherein the use of vanadium, which has an environmental impact, is
minimized. At higher concentrations of V.sub.2O.sub.5 there is
found to be an increased chance of the occurrence of gas bubbles in
an enamel layer formed from the enamel composition, particularly
when used on a substrate layer in which manganese is present as
trace element. Such gas bubbles are undesirable because they have
an unfavourable effect on the mechanical and electrical properties
of the enamel layer.
[0009] It is advantageous if the enamel composition is
substantially free of lithium and sodium ions. Such an enamel
composition is found to be particularly suitable for application as
electrically insulating enamel layer with a high breakdown voltage.
An enamel layer with such an enamel composition, in contrast to
known enamel compositions, is moreover found to be able to
withstand prolonged load with high voltages. This is particularly
important when such an enamel composition is used as dielectric for
a heating elements wherein an electrical resistance is attached to
an enamel layer in order to generate heat. According to regulations
of certifying organizations such as KEMA and ISO, it is desirable
that the enamel composition for such applications can withstand
voltages of 500-3000 V alternating current. When an electrical
potential difference is applied over an enamel composition with
such a composition, the leakage current through the enamel layer at
increased temperatures (for instance greater than 300.degree. C.)
is found to be relatively small compared to known enamel layers.
This results in a relatively long lifespan of the enamel layer at
increased temperatures, such as for instance in heating
elements.
[0010] In a preferred embodiment the enamel composition comprises
between 0.1 and 6% by weight of potassium. Through the addition of
potassium the load-bearing capacity of the adhesion of the enamel
composition to a substrate surface is less critical. In an assembly
of such an enamel composition with a substrate surface less
deformation occurs at increased temperatures, particularly in the
case of overheating. This is particularly advantageous when the
enamel composition is being burnt onto a heating element. The
compressive stress is reduced but is still high enough to prevent
undesired formation of hair cracks. At percentages of potassium
higher than 6% by weight however, there is found to be a greater
chance of formation of hair cracks. In combination with the absence
of other alkali metal ions, in particular lithium and sodium, a low
leakage current also remains guaranteed at increased temperatures.
At increased temperature lithium and sodium have a certain mobility
within the enamel layer which results in electrical conduction.
[0011] The invention also provides an assembly of a substrate
surface and a layer of an enamel composition according to the
invention applied to the substrate surface. An enamel layer
supported by the substrate is thus obtained which can be used as
dielectric.
[0012] The substrate surface is preferably manufactured
substantially from metal. Metal can for instance be used as
current-conducting and/or heat-conducting layer. in combination
with the dielectric layer according to the invention. The metal can
for instance be steel or aluminium.
[0013] It is advantageous if the substrate surface is manufactured
from a material with a coefficient of expansion which differs by
between 20 and 45% from the coefficient of expansion of the enamel
layer. A particular assembly is thus obtained which is found to be
able to withstand temperature changes exceptionally well compared
to known assemblies. Particularly the formation of hair cracks in
the enamel layer has been found to be much less in the case of the
enamel composition according to the invention. With a difference in
coefficient of expansion of less than 20% the chance of hair cracks
becomes too great. The coefficient of expansion can be adapted to
the coefficient of expansion of the substrate by varying the enamel
composition. The content of alkali metal ions (Li, Na, K) of the
enamel composition can be increased in order to increase the
coefficient of expansion. Increasing the potassium content is
recommended here because the leakage current is hereby hardly
affected at increased temperature. In addition to an increase in
the coefficient of expansion, sodium and in particular lithium ions
also result in an increased leakage current at high temperatures.
Conversely, it is of course also possible to opt for a different
substrate. The substrate surface is more preferably manufactured
from a material with a coefficient of expansion which differs by a
maximum of 35% from the coefficient of expansion of the enamel
layer. A difference of between 20-35% in coefficient of expansion
prevents the danger of deformation of the assembly as a result of
overheating.
[0014] It is advantageous if the substrate surface substantially
consists of ferritic chromium steel with a chromium content of at
least 10% by mass. Such an assembly is found to be particularly
impact-resistant compared to known assemblies, and is also
particularly well able to withstand temperature changes.
[0015] In a preferred embodiment the enamel layer applied to the
substrate surface is provided on the side remote from the substrate
layer with a second enamel layer, wherein the second enamel layer
has a relatively higher concentration of alkali metal ions than the
enamel composition of the enamel layer applied to the substrate
surface. It is particularly the concentration of lithium and/or
sodium ions which is important for the concentration, and to a
lesser extent the concentration of potassium ions since potassium
ions contribute less toward leakage current through the dielectric.
The leakage current through the layer between the sensor and an
electrical resistance arranged on one of the layers is preferably
measured prior to heating. In such an assembly the electrical
conductivity of the layer with the higher concentration of alkali
metal ions will be stronger and increase from a lower temperature
than in the case of the layer with the lower concentration of
conductive ions. Such an assembly is particularly useful in
applications such as temperature-sensitive circuit in for instance
a heating element.
[0016] In a particular preferred embodiment there can be arranged
between the enamel layer applied to the subsume surface and the
second enamel layer an electrical sensor for detecting leakage
current through at least one of the layers, normally the layer
having the highest electrical conduction at increased temperature.
The sensor more preferably extends over the whole surface of the
heating element, so that leakage current can be measured at all
positions on the enamel layer. For this purpose the sensor can take
the form of an electrically conductive network or grid. Apart from
alkali metal ions, both layers are preferably manufactured from
substantially the same enamel composition. Optimal adhesion between
the layers is thus obtained. The difference in coefficient of
expansion of the layers is moreover relatively small, so that the
mechanical stresses in the material and adhesion are minimized,
which results in an improved durability.
[0017] It is advantageous if the enamel layer applied to the
substrate surface is substantially free of alkali metal ions.
Particularly understood here by alkali metals are lithium and
sodium. While potassium is also an alkali metal, it is found to
have significantly less influence on the electrical resistance of
the enamel composition. Such an enamel layer retains a relatively
high electrical resistance at increased temperatures, which results
in a relatively small leakage current when a potential difference
is applied over the layer. The layer is moreover found to better
able to withstand high voltages. This increases the accuracy of
leakage current measurements through the second layer. Furthermore,
a difference in resistance between the layers at increased
temperature can be easily realized in such a manner.
[0018] The invention also provides a heating element comprising an
assembly according to the invention. Due to the high temperature
resistance of the enamel layer and the low leakage current the
layer remains intact when the heating element fails as a result of
overheating. In the case of such an overload a very large amount of
heat is released and great electric currents can occur through the
layers. In contrast to known enamel layers, the enamel layer
according to the invention still provides after overload a good
temperature resistance and good electrical insulation properties at
higher temperatures. When such an enamel layer is used in for
instance a heating element in a water boiler, this provides an
improved safety for the user. In existing water boilers with known
enamel layers much more expensive and technically more complicated
measures have to be taken to achieve a comparable safety level. By
making use of an enamel composition according to the invention an
exceptionally safe heating element can thus be constructed in
simple manner. Such a heating element can hereby also be supplied
more cheaply.
[0019] The invention also provides the use of an enamel composition
according to the invention for applying an enamel layer to a
substrate surface. The enamel composition according to the
invention can be applied in known manner to a substrate such as a
metal surface.
[0020] The invention will now be elucidated on the basis of the
following examples.
EXAMPLE 1
[0021] In this example two known enamel compositions A and B are
compared with an enamel composition C according to the invention,
wherein the components in percentages by mass arm shown in table 1.
Using this enamel composition an enamel layer can be prepared and
applied to a substrate in accordance with known methods, such as
described in for instance Petzold and Poschmann, "Email und
Emailliertechnicek" (July 2003).
TABLE-US-00001 TABLE 1 Component A B C Li.sub.2O 6 3 -- K.sub.20 6
7 -- CaO 13 11 28 BaO 6 4 -- ZnO 9 7 10 Al.sub.2O.sub.3 1 1 10
B.sub.2O.sub.3 15 16 8 SiO.sub.2 43 51 46 TiO.sub.2 2 3 --
ZrO.sub.2 1 1 6 PbO -- -- 6 V.sub.2O.sub.5 -- -- 2
[0022] The enamel composition is obtained by mixing the different
raw materials using known rotating melting methods, wherein a glass
frit results after cooling. This glass frit can be finely ground to
a paste for further applications. The obtained paste can for
instance be sprayed onto a substrate, such as a steel surface,
wherein the enamel layer is formed on the substrate surface by
heating.
[0023] Table 2 shows several important physical properties of
enamel layers obtained with the above described enamel
compositions. In this table the coefficient of expansion is
ADK.times.10.sup.-7/K.
Tg (.degree. C.) is the glass transition temperature of the enamel
composition, Te (.degree. C.) is the melting temperature of the
enamel composition, Compressive stress (.times.10.sup.8 Pa): the
compressive stress after cooling of a layer formed from the enamel
composition on a substrate with a coefficient of expansion of
110.times.10.sup.-7/K, such as for instance ferritic chromium
steel, and Burn-out temperature (.degree. C.) is the burn-out
temperature of the enamel layer.
[0024] R.sub.specific (M.OMEGA./m) is the specific electrical
resistance of the enamel layer.
TABLE-US-00002 Property A B C ADK .times. 10.sup.-7/K 265 238 194
T.sub.g (.degree. C.) 495 518 663 T.sub.e (.degree. C.) 535 563 728
Compressive stress (.times.10.sup.8 Pa) 0.69 1.16 2.33 Burn-out
temperature (.degree. C.) 870 870 920 R.sub.specific (M.OMEGA./m)
0.03 0.37 320 crack formation + + -
[0025] The coefficient of expansion is lower than in the known
enamel compositions. A higher compressive stress is also measured
in application on a substrate. The glass transition temperature and
the melting temperature of enamel composition C are also higher
than the known compositions, whereby the integrity and the
temperature resistance of the enamel layer are improved. In
addition, an enamel layer in accordance with composition C is found
to result in less hair crack formation during use. A lower leakage
current is also measured when the layer thickness and voltage are
the same, which is explained by a higher electrical resistance and
a higher breakdown voltage at increased temperatures. The above
factors contribute toward an improved durability during operation
under variable temperatures, as in a heating element.
EXAMPLE 2
[0026] FIG. 1a shows a heating element 1 according to the
invention, wherein the different stacked layers 2, 3, 4, 5, 6 are
shown separately of each other for the sake of clarity. Heating
element 1 comprises a metal substrate layer 2 manufactured from
ferritic chromium steel with a content of 18% by mass of chromium.
Arranged on substrate layer 2 is a first enamel layer 3 according
to the invention, substantially the same as enamel composition C of
the above example, with the difference that 3% by mass of potassium
ions is added to layer 3 in the form of potassium oxide (K.sub.2O).
The enamel layer thereby has a high electrical resistance, also at
high temperatures (for instance higher than 200.degree. C.),
whereby a possible leakage current through the layer 3 is small.
The added potassium ions have a stress-reducing effect on the
compressive stress on enamel layer 3, and increases the coefficient
of expansion of the enamel, whereby less high demands are made of
the adhesion to the substrate. The compression strength does
however remain high enough to counter the formation of hair cracks.
Arranged on the electrically insulating enamel layer 3 is an
electrically conductive grid 4 manufactured from ruthenium oxide
(RuO.sub.2) to which is then applied a second enamel layer 5
according to the invention. The enamel composition of the second
enamel layer is substantially the same as the first enamel layer 3,
with the difference that the second enamel layer 5 has an
electrical resistance which decreases at a lower temperature than
the electrical resistance of insulating layer 3. This effect is
achieved by adding lithium or sodium ions instead of potassium ions
to the enamel composition of the second layer 5, for instance in
the form of the relevant alkali metal oxides Li.sub.2O or
Na.sub.2O. Subsequently arranged on the second layer 5, which has a
relatively better conduction, is an electrical resistance which can
be used to generate heat. In order to monitor the temperature of
heating element 1 during use, sensor layer 4 provides the option of
determining the leakage current through the relatively conductive
layer 5 and/or the relatively insulating layer 3. The magnitude of
the leakage current is indicative of the magnitude of the highest
temperature at a position on the element. Electrical circuits which
can be used for this purpose are described for instance in WO
0167818. The advantage of the use of enamel compositions according
to the invention in such leakage current overheating safety devices
is that a heat element can be obtained with improved durability.
Because substantially the same composition is used for both enamel
compositions 3, 5, an exceptionally good adhesion is obtained
between the two enamel layers 3, 5, wherein the sensor grid 4 makes
good contact with the layers to enable optimum recording of the
leakage current.
[0027] The temperature-dependent progression of the electrical
resistances R of the relatively conductive layer 5 (R.sub.5) and
the relatively insulating layer 3 (R.sub.3) are shown in FIG. 1b.
It can be seen that, as temperature T increases, the resistance of
the relatively conductive layer 5 (R.sub.5) decreases more rapidly
and at a lower temperature than the resistance (R.sub.3) of the
relatively insulating layer 3. The breakdown voltage of the
relatively insulating layer 3 is thus significantly higher at
increased temperatures than the breakdown voltage of the relatively
conductive layer 5. By modifying the composition of the enamel
compositions to the desired minimum and maximum heating
temperature, a temperature safety device for heating element 1 can
be realized using a relatively simple electrical circuit.
[0028] It will be apparent that for a skilled person in the field
many applications and variations of the enamel composition
according to the invention can still be envisaged.
* * * * *